Conventionally, a control device for a multi-phase AC motor with three or more phases has been known. The control device includes inverters in two systems corresponding to the pair of winding wire sets forming the multi-phase AC motor and controls current supply to each of the pair of winding wire sets. For example, the control device described in Patent Literature 1 includes, as a mechanical configuration, a motor having a pair of 3-phase winding wire sets at 30° angular intervals held therebetween, and 3-phase inverters in two systems which are connected to the pair of 3-phase winding wire sets. By applying 3-phase AC voltages having a phase difference therebetween to the pair of 3-phase winding wire sets, a reduction in torque ripple (pulsation) in the motor is achieved.
Among various AC motors mounted in vehicles, a column-mounted steering assist motor used in an electric power steering device is placed at a position particularly close to a driver. Accordingly, when a torque ripple is caused by a harmonic component superimposed on the fundamental sine wave of the AC voltage output from an inverter, not only vibration is applied to a steering wheel, but also the sound resulting from the torque ripple may be unpleasant to the driver's ear.
To prevent this, it has been known to use the technique disclosed in Patent Literature 1 or the like and provide the 3-phase AC voltages applied to the pair of winding wire sets with a phase difference of a 30° electric angle. This can mainly reduce fifth- and seventh-order harmonic components.
However, the sound which causes an uncomfortable feeling in the driver may be affected not only by the fifth- and seventh-order components, but also by an eleventh-, thirteenth-, or higher-order harmonic component depending on the relationship with the resonant frequency of the multi-phase AC motor mounted in the vehicle or the like. In addition, a person may feel that the sound heard when a given frequency distribution state steadily continues is more unpleasant to his ear than the sound heard when a specified-order harmonic component is momentarily generated.
Thus, when attention is focused on the problem of the “sound unpleasant to the driver's ear”, the foregoing problem cannot completely be solved by merely fixing the phase difference between the AC voltages applied to the pair of 3-phase winding wire sets to 30° and reducing the torque ripple derived from the fifth- and seventh-order components.